Global evaluation of Doppler velocity errors of EarthCARE Cloud Profiling Radar using global storm-resolving simulation
Abstract. The Cloud Profiling Radar (CPR) on the Earth Clouds, Aerosol, and Radiation Explorer (EarthCARE) satellite is the first satellite-borne Doppler radar (EC-CPR). In our previous study, we examined the effects of horizontal (along-track) integration and simple unfolding methods on the reduction of Doppler errors in the EC-CPR observations, and those effects were evaluated using two limited scenes in limited latitude and low pulse repetition frequency (PRF) settings. In this study, the amount of data used was significantly increased, and the area of the data used was extended globally. Not only low PRF but also high PRF settings were examined. We calculated the EC-CPR-observed Doppler velocity from pulse-pair covariances using the radar reflectivity factor and Doppler velocity obtained from a satellite data simulator and a global storm-resolving simulation. The global data were divided into five latitudinal zones, and mean Doppler errors for 5 dBZe after 10 km integration were calculated. In the case of low PRF setting, the error without unfolding correction for the tropics reached a maximum of 2.2 m s-1 and then decreased toward the poles (0.43 m s-1). The error with unfolding correction for the tropics became much smaller at 0.63 m s-1. In the case of high PRF setting, the error without unfolding correction for the tropics reached a maximum of 0.78 m s-1 and then decreased toward the poles (0.19 m s-1). The error with unfolding correction for the tropics was 0.29 m s-1, less than half the value without the correction. The results of the analyses of the simulated data indicated that the zonal mean frequency of precipitation echoes was highest in the tropics and decreased toward the poles. Considering a limitation of the unfolding correction for discrimination between large upward velocity and large precipitation falling velocity, the latitudinal variation of the Doppler error can be explained by the precipitation echo distribution.
Yuichiro Hagihara et al.
Yuichiro Hagihara et al.
Yuichiro Hagihara et al.
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This paper is concerned with the accuracy of the vertical velocity observations that should be achieved by the nadir pointing 94GHz Doppler radar on the EarthCARE satellite when it is launched. It is an extension of the estimates in an earlier paper (HH, 2022) that used the same NICAM global model with 3.5km horizontal resolution to forward model the values of radar reflectivity and vertical velocity over 2 orbits with a prf close to 6100Hz so the folding velocity is +/- 4.8m/s so that precipitation with a terminal velocity of 6m/s would be folded and appear as an upward velocity of - 3.6m/s (the convention is that downward velocity is positive). Such upward velocities were deemed unlikely and so any upward velocity above 3m/s was unfolded by subtracting twice the folding velocity. When velocities were unfolded then the average standard deviation of the retrieved vertical velocity was reduced.
The advance in this paper is to extend the analysis firstly to 16 orbits (rather than 2 orbits in the HH paper) so that the degree of folding as a function of latitude can be found, and secondly to extend the analysis to 7500Hz so the folding velocity is 6m/s rather than 4.8m/s for 6100Hz. The advantage of the 7500Hz frequency is a reduction in the phase noise due to the reshuffling of the target caused by a Doppler width of the target assumed to be 4m/s due to satellite motion and a finite beamwidth. This leads to a lowering of the correlation between two sequential pulses (Equn 2 in this paper, equn 3 in HH).
The conclusion of this new paper (abstract lines 21-24) is:
“…the mean frequency of the precipitation echoes was highest in the tropics and decreased toward the poles. Considering a limitation of the unfolding correction for discrimination between large upward velocity and large precipitation falling velocity, the latitudinal variation of the Doppler error can be explained by the precipitation echo distribution.”
The conclusion of the new paper in the abstract lines 21-24 that heavier precipitation occurs in the tropics is to be expected, but it would be useful to know the values of reflectivity and the type of particles that are needed to produce the high terminal velocities above 6 m/s at 94GHz, 7500Hz and consequent folding. In most rainfall Mie scattering of the larger drops at 94GHz leads to terminal velocities much below 6 m/s.
The authors appear to have neglected the effect of multiple scattering which leads to very noisy phase returns due to the differing path lengths of the multiply scattered photons. This effect becomes important for rain rates above 5 mm/hr and will drastically degrade the quality of the Doppler, see Matrosov et al. 2008. https://doi.org/10.1175/2008JTECHA1095.1
The model has a resolution of 3.5km so the size of the features that can be represented is probably greater than 10km. This means that the forward modeled values of reflectivity for each km in the horizontal will not be independent but will be smoothed, and secondly the full range of updrafts and downdrafts will not be resolved. In the current analysis based on the NICAM model it is assumed that any updraft above 3m/s is deemed to be unlikely, but in reality updrafts much higher than this do occur on the km or sub km scale.
Less important issues with the presentation. The paper relies too much on quoting results from HH. It would read better is some explanations were provided.